Transformer core with composite offset V-miter and step joint

ABSTRACT

A step, or butt joint is combined with a V-miter joint to improve the ability of magnetic flux to transition from a yoke member to an inner leg member, or vice versa, to minimize the number of different shaped layers required to form a leg member and to improve inner leg member to yoke member joint strength in a transformer core of the stacked flat-layer type. The V-miter portion of the joint permits the use of a single piece yoke member and the step, or butt portion of the joint reduces joint air gap and therefore joint magnetic reluctance. Structural rigidity at an inner leg member to yoke member joint is improved over that of a transformer core with a yoke having two or more pieces and all layers of said inner leg member have the same shape.

United States Patent 1 1 [111 3,895,336

Pitman 1 July 15, 1975 TRANSFORMER CORE WITH COMPOSITE 3,743,991 7/1973 Gumpper et al 336/217 OFFSET V-MITER AND STEP JOINT [75] Inventor: Frank A. Pitman, Rome, Ga.

[73] Assignee: General Electric Company,

Pittsfield, Mass.

[22] Filed: June 24, 1974 [21] App]. No; 482,053

[51] Int. Cl. H011 27/24 [58] Field of Search 336/216. 217, 214, 215; 29/609; 310/217 [56] References Cited UNITED STATES PATENTS 2.467.823 4/1949 Gordy 336/217 X 3.210.708 10/1965 Franklin 6. 336/217 3,290,633 12/1966 Kuriyama et a1. 11 336/217 X 3,303,448 2/1967 Farry 336/2l7 X 3,504,318 3/1970 Wilburn et al. 336/217 X Primary Examiner-Thomas J. Kozma Attorney, Agent, or Firm.lohn J. Kelleher [57} ABSTRACT A step, or butt joint is combined with a V-miter joint to improve the ability of magnetic flux to transition from a yoke member to an inner leg member, or vice versa, to minimize the number of different shaped lay ers required to form a leg member and to improve inner leg member to yoke member joint strength in a transformer core of the stacked flat-layer type. The V-miter portion of the joint permits the use of a single piece yoke member and the step, or butt portion of the joint reduces joint air gap and therefore joint magnetic reluctance. Structural rigidity at an inner leg member to yoke member joint is improved over that of a transformer core with a yoke having two or more pieces and all layers of said inner leg member have the same shape.

4 Claims, 4 Drawing Figures a 138 X I 180 1" 1/54 186" I80 v40 18+ 108 448 I74 (78 164 m? 17a. 1 5' 170 1684 14e- TRANSFORMER CORE WITH COMPOSITE OFFSET V-MITER AND STEP JOINT BACKGROUND OF THE INVENTION The present invention relates to magnetic core structures for electrical induction apparatus and more particularly to a joint formed between an inner leg member and a yoke member of a stacked flat-layer type transformer core having at least three legs.

In transformer cores of the stacked flat-layer type, cores are normally formed from magnetic strip material. The magnetic strip material in each layer is generally made of highly grain-oriented and cold-rolled silicon alloy steel cut to length from a rolled, or coiled strip of such material. This magnetic strip material has a most favorable magnetic direction corresponding to its lengthwise axis which is also the normal direction of magnetic flux through the strip material.

Each layer of a stacked flat-layer type transformer core is formed of leg members and yoke members which are joined together by bevel, or miterjoints, butt joints or various combinations of such joints. The bevel shape at the joint between yoke member and leg member is intended, in the case of grain-oriented magnetic strip material with a preferential magnetic direction, to deflect the flux from leg to yoke, or vice versa, with a minimum of loss. In some cases a joint provides a degree of structural rigidity.

It is well known that the greater the number ofjoints between leg members and yoke members the greater the number of air gaps due to these joints and therefore the greater the magnetic reluctance in the transformer core. The greater the magnetic reluctance of a core the lower its efficiency. It is fairly common practice in constructing transformer cores to make a layer portion of a leg member of single-piece construction and to make a layer portion of a yoke member of two or more piece construction. Single-piece yoke members have been used but to a lesser degree. An example of a transformer core incorporating a single-piece layer portion of a yoke member can be seen in U.S. Pat. No. 3,153,215 to BURKHARDT et al. An example of a two-piece layer portion of a yoke member can be seen in US. Pat. No. 3,303,448 to FARRY.

in forming the joint between an inner leg member and a yoke member two general design approaches have been utilized. In one design, the ends of an inner leg member are beveled and a joint is formed between a beveled end of an inner leg member and a two-piece yoke member as in the aforementioned FARRY patent. However, this type of design involves more magnetic reluctance increasing joints than a single-piece yoke. In FARRY a step portion extends laterally from the beveled end portion of the center leg member but the twopiece yoke design unnecessarily decreases core efficiency and does not allow as much structural rigidity to be utilized from the use of such a step portion, than if the layer portion of the yoke members were of singlepiece design. In another design, as described in the aforementioned BURKHARDT et al. patent, the layer portions of the yoke member are of single-piece design with a V-shaped notch extending outwardly toward an inner side thereof. The ends of the layer portion of the inner leg member are V-shaped for mating cooperation with the V-shaped notch in the aforementioned yoke member to form a V-miter joint. For structural support and air gap bridging reasons, the central axis of the V- shaped notch in a yoke member is offset from the longitudinal axis of an inner leg member. When this was done in prior art designs, such as in BURKHARDT et al., less than complete mating between an end of an inner leg member and a yoke member resulted. This arrangement created a relatively large air gap and therefore is a less efficient joint than a joint where the V- shaped notch in a yoke member is completely filled with the V-shaped end of a mating inner leg member and where a laterally extending step or butt joint is added to the V-miter joint. Additionally, inner leg members in the aforementioned transformer cores are normally constructed of layers that are formed by several layers having different shapes. Fabrication of such inner leg members is a costly and time consuming task.

In order to avoid these and other disadvantages it would be desirable to provide an inner leg member to yoke member joint that will minimize the air gap at the joint between an inner leg member and a yoke member, minimize the number of yoke member to inner leg member joints and minimize the number of different shaped layers required to form an inner leg member of a stacked flat-layer type transformer core having at least three legs.

Accordingly, it is an object of the present invention to provide an inner leg member to yoke member joint for a transformer core of the stacked flat-layer type having at least three legs that will minimize the air gap and therefore the magnetic reluctance at said inner leg member to yoke member joint.

It is a further object of the present invention to provide an inner leg member to yoke member joint for a transformer core of the stacked flat-layer type having at least three legs that will minimize the number of joints at the interface between an inner leg member and a yoke member.

Still another object of my invention is to provide a transformer core of the stacked flat-layer type having at least three legs that will minimize the number of different shaped layers required to form an inner leg member.

SUMMARY OF THE INVENTION An improved inner leg member to yoke memberjoint in a layered transformer core having at least three legs is disclosed. A step, or butt joint is combined with a V- miter joint to improve joint strength and the ability of magnetic flux to transition from a yoke member to an inner leg member, or vice versa, of a layered transformer core. The V-miter portion of the yoke member to inner leg member joint permits the use of a singlepiece yoke member and the step, or butt portion of such a joint reduces the joint air gap and therefore the joint magnetic reluctance. In addition, structural rigidity at said inner leg member to yoke memberjoint is improved over that ofa transformer core with a yoke having two or more pieces and all layers of said inner leg member have the same shape.

The invention, which is sought to be protected, will be particularly pointed out and distinctly claimed in the claims appended thereto. However, it is believed that this invention and the manner in which its objects and advantages are obtained, as well as other objects and advantages thereof will be more readily understood by reference to the following detailed description of the preferred embodiment thereof particularly when considered in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation of a transformer core incorporating the offset V-miter and step joint of the present invention.

FIG. 2 is a fragmented perspective view of a miter joint between an outside leg member and a yoke member of the transformer core of the present invention.

FIG. 3 is a fragmented and exploded perspective view of a two-lamination transformer core layer incorporating the V-miter and step joint of the present invention.

FIG. 4 is a fragmented and exploded perspective view of a two-lamination layer of an inner leg member showing the V-shaped ends of said two-lamination layer offset in opposite directions.

DESCRlPTlON OF THE PREFERRED EMBODlMENT Referring now to the drawings wherein like numerals are used to indicate like parts throughout, in FIG. 1 a front elevation of three-legged multi-layer transformer core 120 of the stacked flat-layer type incorporating the preferred embodiment of the present invention is illustrated. Core 120 is comprised of outer leg members 122 and 124, inner leg member 126 and yoke members 128 and 130, all of said core members 122, 124, 126, 128 and 130 having the same width. Leg members 122 and 124 are generally rectangular in shape and are parallel to and equally spaced from the generally rectangular shaped inner leg member 126. Yoke members 128 and 130 are generally rectangular in shape and are oriented such that their longitudinal axes are perpendicular to the longitudinal axes of leg members 122, 124 and 126. Yoke members and leg members are formed of laminated layers of magnetic strip material which can best be seen by referring to FIG. 2. In FIG. 2 which is a fragmented perspective view of a miter joint between a yoke member and a leg member, a core having a plurality of laminated layers is illustrated. Each layer is made up of two identically shaped laminations. A layer 132, for example, is comprised of identical laminations 134 and 136. Each of the remaining layers also contain two identical laminations. However, all of the laminations in all of the layers of a yoke member or outer leg member are not identical. Each laminated layer is fabricated from highly grain-oriented and coldrolled silicon alloy steel. The yoke and leg members have a most favorable magnetic direction that corresponds to the longitudinal axes of said members.

Referring again to FIG. 1, an end of a layer portion of a yoke member is beveled and engages a beveled end of a layer portion of a leg member by means of a miter joint having a small step portion. A layer of leg member 122 engages a co-planar layer of yoke member 128 along miter joint 138 which has a small step portion 140. An immediately adjacent layer of leg member 122 engages a co-planar layer in yoke member 128 along miter joint 142 which has a small step portion 144. As can be seen, joint 138/ l 40 is offset from joint 142/l44. This offset is accomplished by lengthening a leg member layer and shortening a yoke member layer, or vice versa, in the region where these layers are joined. This alternate offset arrangement is repeated throughout the entire joint between yoke member 128 and leg member 122. By offsetting the joints in these adjacent layers, the air gap formed in such a miter joint is bridged or spanned by an adjacent layer which lowers the magnetic reluctance caused by such an air gap. In a like manner, a layer of leg member 122 engages a co-planar layer of yoke member along miter joint 146 which has a small step portion 148. An immediately adjacent layer of leg member 122 engages a co-planar layer in yoke member 130 along offset miter joint 150 which has a small step portion 152. This alternate offset feature is repeated throughout the entire joint between yoke member 130 and leg member 122. Also in a like manner, a layer of leg member 124 engages a co-planar layer of yoke member 128 along miter joint 154 which has a small step portion 156. An immediately adjacent layer of leg member 124 engages a co-planar layer of yoke member 128 along offset miter joint 158 which has a small step portion 160. This alternate offset feature is repeated throughout the entire joint between yoke member 128 and leg member 124. Also in like manner, a layer ofleg member 124 engages a co-planar layer of yoke member 130 along miter joint 162 which has a small step portion 164. An immediately adjacent layer of leg member 124 engages a co-planar layer of yoke member 130 along offset miter joint 166 having a small step portion 168. This alternate offset feature is repeated throughout the entire joint between yoke member 130 and leg member 124. The angle of a miter joint between a yoke member and an outer leg member is preferably 45.

With continuing reference to the FIG. 1, the joint between inner leg member 126 and yoke member 130 is illustrated incorporating the V-miter and step joint of the present invention. lnner leg member 126 of generally rectangular shape has the same number of layers and laminations per layer as do leg members 122 and 124 and yoke members 128 and 130. Every layer in leg member 126 is identical in shape. The longitudinal center axis of inner leg member 126 is equidistant from the longitudinal center axes of said outer leg members 122 and 124. An end of a layer portion of an inner leg member engages a side of a layer portion of a yoke member by means of the V-miter and step joint of the present invention. The V-miter portion 170 of the V-miter and step joint consists of a symmetrically pointed. or V- shaped end portion of inner leg member 126 that has its central axis parallel to and offset from the longitudinal center axis of said inner leg member 126. The internal angle of said symmetrically pointed, or V-shaped end portion is preferably 90. A layer of the V-shaped end portion of leg member 126 is cooperatively engageable with a V-shaped cut in a co-planar layer of yoke member 130. The V-shaped cut in said layer portion of yoke member 130 is outwardly opening toward an internal side of yoke member 130. The V-shaped cut has an internal angle of 90 and the central axis of the V-shaped cut is perpendicular to the longitudinal axis of yoke member 130.

A side 172 of yoke member 130 is parallel to the lon gitudinal axis of said yoke member 130. The V-shaped cut in yoke member 130 is the same size as the V- shaped end portion of leg member 126 and therefore full engagement of the V-shaped end portion of leg member 126 with the V-shaped cut in yoke member 130 completely fills the \/-shaped cut in said yoke member 130. The V-shaped end portion of inner leg member 126 does not extend beyond side 172 of yoke member 130 when said V-shaped end portion is fully engaged. The just mentioned type of engagement between inner leg member 126 and yoke member 130 is here defined as cooperative engagement.

In addition to V-miter portion 170 of the V-miter and step joint, said V-miter and step joint further consists of a small step portion 174 that extends laterally from a side of the V-shaped end portion of inner leg member 126 in a direction that is perpendicular to the longitudinal axis of said inner leg member 126. Astep portion such as step portion 174 always extends in a direction that is opposite to the direction of the Vs haped end portion offset associated therewith. When a V-shaped end of inner leg member 126 is fully inserted in a V- shaped cut in yoke member 130, step portion 174 of the V-miter and step joint, being parallel to side 172 of yoke member 130, engages said side 172 in a butting relationship. This just described butting engagement between inner leg member 126 and yoke member 130 minimizes the air gap and therefore the magnetic reluctance between these two members. An immediately adjacent layer in inner leg member 126 engages a coplanar layer in yoke member 130 along a V-miter and step joint consisting of V-miter portion 176 and step portion 178. V-miter and step joint 176/178 is identical to V-miter and step joint 170/174 except that the V- miter in one layer is offset in a direction that is opposite to that in an immediately adjacent layer. Immediately adjacent layers in inner leg member 126 have the V- miter and step joints at the ends thereof alternately offset in opposite directions throughout the entire thickness of said inner leg member 126.

With continuining reference to FIG. 1 a V-miter and step joint having the same shape as that between inner leg member 126 and yoke member 130 is formed in the same way between inner leg member 126 and yoke member 128. The V-miter portion 180 of the V-miter and step joint between inner leg member 126 and yoke member 128 consists of a symmetrically pointed, or V- shaped end portion of inner leg member 126 that has its central axis parallel to and offset from the longitudinal center axis of said inner leg member 126. The internal angle of said V-shaped end portion is preferably 90. A layer of the V-shaped end portion ofleg member 126 is cooperatively engageable with a V-shaped cut in a co-planar layer of yoke member 128. The V-shaped cut in said layer portion of yoke member 128 is outwardly opening toward an internal side of said yoke member 128. The V-shaped cut has an internal angle of 90 and the central axis of the V-shaped cut is perpendicular to the longitudinal axis of yoke member 128.

A side 182 of yoke member 128 is parallel to the longitudinal axis of said yoke member 128. The V-shaped cut in yoke member 128 is the same size as the V- shaped end portion of leg member 126 and therefore full engagement of the V-shaped end portion of leg member 126 with the V-shaped cut in yoke member 128 completely fills said V-shaped cut in said yoke member 128. The V-shaped end of inner leg member 126 does not extend beyond side 182 of yoke member 128 when said inner leg end portion is fully inserted into said V-shaped cut in yoke member 128.

In addition to V-shaped portion 180 of the V-miter and step joint, the V-miter and step joint further consists of a small step portion 184 that extends laterally from a side of the V-shaped end portion of inner leg member 126 in a direction that is perpendicular to the longitudinal axis of said inner leg member 126. When a V-shaped end of inner leg member 126 is fully inserted in a V-shaped cut in yoke member 128,,step portion 184 of the V-miter and step joint, being parallel to side 182 of yoke member 128, engages said side 182 in a butting relationship. An immediately adjacent layer in inner leg member 126 engages a co-planar layer in yoke member 128 along a V-miter and step joint consisting of V-miter portion 186 and step portion 188. V- miter and step joint 186/188 is identical to V-miter and step joint /184 except that the V-miter in one of said joints is offset in a direction opposite to that of a V-miter in a V-miter and step joint of an immediately adjacent layer. Immediately adjacent layers in inner leg member 126 have the V-miter and step joints at the ends thereof alternately offset in opposite directions throughout the entire thickness of said inner leg member 126.

As previously stated, all of the aforementioned transformer core leg members and yoke members have the same number of layers. All of these layers are laminated with each layer having two laminations. A laminated layer of the type used to form the V-miter and step joint between inner leg member 126 and yoke member 130 or between inner leg member 126 and yoke member 128 is illustrated in FIG. 3. Referring now to FIG. 3, an exploded and perspective view of the aforementioned type joint shows a laminated layer of inner leg member 126, consisting of lamination 190 and 192, oriented for engagement with a laminated layer of a yoke member which consists of lamination 194 and 196.Lamination 190 ofinner leg member 126 is identical in every respect to lamination 192 of leg member 126. In addition, lamination 194 of a yoke member is identical in every respect to lamination 196 of said yoke member. FIG. 4 is a fragmented and exploded perspective view of a layer of inner leg member 126, consisting of laminations 198 and 200 oriented for engagement with a layer of a yoke member which consists oflaminations 202 and 204. In FIG. 4 however, the V-miter portion ofthe illustrated V-miter and step joint is offset in a direction opposite to the V-miter offset in FIG. 3. This offset feature provides magnetic reluctance reducing bridging of an immediately adjacent air gap in a V-miter and step joint as well as structural strength at said V-miter and step joint.

It should be noted that even though immediately adjacent layers of inner leg member 126 are offset in opposite directions, the longitudinal center axis of each layer remains equidistant from the longitudinal center axes of outer leg members 122 and 124.

GENERAL CONSIDERATIONS The V-shaped cut made in the side of a yoke member for cooperative enagement with a V-shaped end of an inner leg member may be made to any depth short of a depth that divides the yoke member into two pieces. However, for optimum transition of magnetic flux between yoke member and inner leg member and for optimum strength at the joint between said inner leg member and yoke member the depth of cut was found to be one-half the width ofa yoke member less one-half the length of the step portion of the V-miter and step joint of the present invention.

In the preferred embodiment, the direction of offset of the V-miter portions at the ends of the inner leg member has been shown to be in the same direction. However, my invention would be applicable to situa- 7 tions where it is desirable to offset the V-miter portion of one end of an inner leg layer in one direction and the V-rniter portion at the opposite end of the same inner leg layer member in the opposite direction. This variation is illustrated in FIG. 4.

Although the angle of 90 is the preferred angle at the yoke member V-shaped cut and at the V-shaped end of an inner leg member, my invention has application where such an angle is greater than or less than 90.

The preferred embodiment describes a core having layers with two laminations. However, my invention would also be applicable to transformer cores having one lamination per layer or having three or more laminations per layer. The only requirement for multilamination layers in this regard, is that all laminations in any one layer be identical.

The term substantial void" used herein means a void other than that which normally exists between yoke and leg members in the region where one such member is in butting engagement with another such member.

I claim:

1. In a transformer core of the stacked fiat-layer type having a plurality of flat, laminated, layered members of equal width, each of said members including,

two generally rectangular shaped, parallel, spacedapart, outside leg members beveled at each end;

two generally rectangular shaped, single piece yoke members beveled at each end, each yoke member having its longitudinal axis perpendicular to the longitudinal axis of said legs forming a rectangular shaped structure, a partially beveled end portion of a yoke member en gaging a partially beveled end portion of a leg member forming a miter joint between a yoke member and a leg member, adjacent miter joints in adjacent layers being offset in the plane of a layer such that a miter joint in one layer is bridged by a portion of an adjacent layer;

a generally rectangular shaped inner leg member, parallel to and intermediate of said outer leg members, an end of said inner leg member engaging a side portion of a yoke member forming an inner leg member to yoke member joint; wherein the improvement comprises said inner leg member to said yoke member joint, said joint comprising:

a. a generally rectangular shaped yoke member having an outward opening V-shaped cut at an internal side thereof; and

b. a generally rectangular shaped inner leg member having V-shaped end portions, the central axis of said V-shaped end portions being parallel to and offset in the same direction from the longitudinal axis of said inner leg member and having a step portion extending laterally from said V-shaped end in a direction that is perpendicular to the longitudinal axis of said inner leg member, whereby a V- shaped end of said inner leg member is cooperatively engageable with a V-shaped cut in said yoke, and said step portion is engageable with a side of said yoke member in a butting relationship such that no substantial voids exist between a yoke member and an inner leg member. adjacent joints in immediately adjacent layers being offset in opposite directions in the plane of a layer, in the direction of the longitudinal axis of a yoke member such that a joint in one layer is bridged by a portion of an adjacent layer.

2. An improved transformer core joint as defined in claim 1 wherein the depth of a V-shaped cut in a yoke member is one-half the width of a yoke member less one-half the length of said step portion of said inner leg member to yoke member portion.

3. An improved transformer core joint as defined in claim 1 wherein the internal angle of the \'-shaped cut in a yoke member and the internal angle of a cooperatively engageable V-shaped end portion of an inner leg member is 4. An improved transformer core joint as defined in claim 1 wherein only one inner leg member is utilized and said inner leg member is equally spaced from the two outer leg members.

* i I t 

1. In a transformer core of the stacked flat-layer type having a plurality of flat, laminated, layered members of equal width, each of said members including; two generally rectangular shaped, parallel, spaced-apart, outside leg members beveled at each end; two generally rectangular shaped, single piece yoke members beveled at each end, each yoke member having its longitudinal axis perpendicular to the longitudinal axis of said legs forming a rectangular shaped structure, a partially beveled end portion of a yoke member engaging a partially beveled end portion of a leg member forming a miter joint between a yoke member and a leg member, adjacent miter joints in adjacent layers being offset in the plane of a layer such that a miter joint in one layer is bridged by a portion of an adjacent layer; a generally rectangular shaped inner leg member, parallel to and intermediate of said outer leg members, an end of said inner leg member engaging a side portion of a yoke member forming an inner leg member to yoke member joint; wherein the improvement comprises said inner leg member to said yoke member joint, said joint comprising: a. a generally rectangular shaped yoke member having an outward opening V-shaped cut at an internal side thereof; and b. a generally rectangular shaped inner leg member having Vshaped end portions, the central axis of said V-shaped end portions being parallel to and offset in the same direction from the longitudinal axis of said inner leg member and having a step portion extending laterally from said V-shaped end in a direction that is perpendicular to the longitudinal axis of said inner leg member, whereby a V-shaped end of said inner leg member is cooperatively engageable with a V-shaped cut in said yoke, and said step portion is engageable with a side of said yoke member in a butting relationship such that no substantial voids exist between a yoke member and an inner leg member, adjacent joints in immediately adjacent layers being offset in opposite directions in the plane of a layer, in the direction of the longitudinal axis of a yoke member such that a joint in one layer is bridged by a portion of an adjacent layer.
 2. An improved transformer core joint as defined in claim 1 wherein the depth of a V-shaped cut in a yoke member is one-half the width of a yoke member less one-half the length of said step portion of said inner leg member to yoke member portion.
 3. An improved transformer core joint as defined in claim 1 wherein the internal angle of the V-shaped cut in a yoke member and the internal angle of a cooperatively engageable V-shaped end portion of an inner leg member is 90*.
 4. An improved transformer core joint as defined in claim 1 wherein only one inner leg member is utilized and said inner leg member is equally spaced from the two outer leg members. 